The research involves the role of the contact between pelvic urine and renal papillary epithelium in the concentration of the final urine. Interstitial fluid concentrations and movements in the renal papilla are studied. The interstitial spaces are studied by dye injections, light and electron microscopy. The structure and function of renal pelvic extensions are studied in mammals on high and low protein diets. Structure and function of the elasmobranch excretory organs are studied by micropuncture and electron microscopy. Studies of cellular volume and ion regulation will be continued in fish muscle and mammalian renal papilla. BIBLIOGRAPHIC REFERENCES: Schmidt-Nielson, Bodil. Intracellular concentrations of the salt gland of the herring gull, Larus argentatus. Am. J. Physiol. 230: 514-521 (1976). Schmidt-Nielson, Bodil, Eric R. Lacy, and Yogendra Patel. Interstitial fluid in the mammalian renal papilla: Concentration, movement and channels. Fed. Proc. 35: 372 (1976).